1. Field of the Invention
This invention relates to infinite impulse response (IIR) filters.
2. Description of the Prior Art
As is known to those skilled in the art, an IIR filter is a type of digital filter which can be used to effect filtration of an input signal that comprises a sequence of digital words (which may, for example, comprise samples of an analog signal) spaced in time by an interval T. The filter is operative during each of a plurality of periods of time (clock periods) equal to T to process sets of relatively delayed words of the input signal to produce a word of a filtered output signal. The IIR filter is distinguished from some other types of digital filter in that it is recursive. That is to say, each output word (or sample) is a function of a past or previously generated word or words as of present and past input words. Accordingly, the IIR filter includes a feedback loop which feeds back at least one word to adding means where it is summed with other signals. In the feedback loop, the word that is fed back is multiplied by a weighting coefficient. The multiplication operation takes time. So does the subsequent addition operation. If the filter is to function, the total of the times taken for addition and multiplication must not exceed T. That is to say, the filter cannot handle an input signal of which the data rate (word or sample frequency) fs is so high that T(=1/fs) is less than the total of the above-mentioned multiplication and addition times. This imposes a severe constraint of the designer of a filter intended to handle a high speed input signal in that even the fastest (and most expensive) multipliers and adding means may not be fast enough.